Diode design to reduce the effects of radiation damage

ABSTRACT

A photodetector for X-ray applications includes a photodiode at each pixel location that is gated to reduce leakage of charge from the photodiode. A gate layer may be disposed around the entire peripheral edge of the detector, and maintained at a common potential with a contact layer, or at a different potential. A passivation or dielectric layer separates the gate layer from the photodiode. Leakage around the edge of the diode that can result from extended exposure to radiation is reduced by the gate layer.

BACKGROUND

The invention relates generally to photodetectors, such as for use inX-ray imaging applications. More particularly, the invention relates toa photodiode design for such detectors that reduces leakage of chargefrom the photodiode that can result from exposure of the photodetectorto radiation over time.

Digital X-ray imaging systems have become increasingly important in anumber of technical areas. Such imaging systems are presently used inmedical applications, such as for projection X-ray, X-ray tomosynthesis,computer tomography systems, and so forth. Digital X-ray systems arealso currently in use for part and parcel inspection, and other suchnon-medical uses. In general, digital X-ray imaging systems rely upon astream of X-ray radiation from a source that impacts a detector arrayafter traversing a subject or object of interest. The X-ray radiation isreceived by a scintillator in the detector, and charges in photodiodesare depleted by photons from the scintillator. The charge depletion canbe measured, resulting in information for discrete picture elements(“pixels”) at the photodiode locations that can be analyzed forreconstruction of an image.

Flat panel amorphous silicon-based X-ray detectors of the type currentlyused in digital X-ray systems have excellent performance, but maydegrade over time. One mechanism of such degradation involves theamorphous silicon photodiode which can become leaky as a function ofX-ray dose. This leakage may be proportional to the area of thephotodiode, the length of the periphery of the photodiode, or acombination of the two.

Where flat panel detectors are used for medical applications, dosagesare typically prescribed based upon exposure limitations of bothpatients and medical staff. In other environments, however, such as forindustrial part inspection, much stronger X-ray doses may be used. Suchstronger doses tend to significantly reduce the life of conventionaldigital X-ray detectors by significantly increasing the leakage. Similardegradation occurs in medical applications, although over longer periodsof time. As the leakage increases, a dark image offset value may bechanged to accommodate the leakage, but this ultimately results inreduction of the dynamic range of the individual pixels. That is, theamount of charge between the dark image charge and the fully exposedcharge becomes reduced, ultimately resulting in decommissioning of thedetectors at the end of an abbreviated useful life.

There is a need, therefore, for an improved digital X-ray detector thatcan avoid problems with degradation of photodiodes due to exposure.There is a particular need for a photodiode design that can operate inconventional X-ray settings, while reducing or limiting leakage and theconsequent degradation of the detector pixels and their performanceowing to such leakage.

BRIEF DESCRIPTION

The present invention provides a novel design for a photodiode anddetector designed to respond to such needs. The photodiode is configuredto receive photons and to generate signals based upon such receipt, suchas through a scintillator that receives X-ray radiation. An amorphoussilicon photodiode is provided at each pixel location of the detector,in an array with similar photodiodes. The amorphous silicon photodiodeis surrounded by a gate layer that limits leakage of charge (i.e.,electrons) around edges of the diode. The gate layer may be formedaround the entire periphery of the diode, and may form a contact for thediode, such as by contacting an upper surface thereof. The gate layermay be used in conjunction with a separate contact layer, and maintainedat either the same potential as the contact layer (e.g., by means of avia connecting the two layers), or at a different potential.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagrammatical representation of an X-ray imaging systemincluding a digital detector made in accordance with aspects of thepresent invention;

FIG. 2 is a plan view of an exemplary pixel of the detector of FIG. 1,illustrating a first exemplary layout having a gate mechanism forreducing leakage;

FIG. 3 is a partial sectional view of a portion of the edge of theexemplary photodiode and its surrounding structures of FIG. 2,illustrating exemplary layers of the photodiode and gate structures;

FIG. 4 is a plan view of a further exemplary configuration of aphotodiode adapted to reduce leakage by reducing the area and peripherallength of the amorphous silicon photosensitive region, and using asurrounding structure as a reflector/contact;

FIG. 5 is a partial sectional view of a portion of the structure shownin FIG. 4, illustrating various layers in the exemplary implementation;and

FIG. 6 is a plan view of a further alternative implementation of thephotodiode at a pixel location that utilizes a diode gate in conjunctionwith a reflector/contact.

DETAILED DESCRIPTION

Turning now to the drawings, and referring first to FIG. 1, an X-rayimaging system 10 is illustrated diagrammatically. The imaging system isdesigned as a projection X-ray system, although other systems may beemployed. For example, the present invention may be used in detectors ofprojection X-ray systems, tomosythesis systems, computed tomographysystems, and so forth.

In the exemplary system illustrated in FIG. 1, a subject 12 ispositioned between a source of X-ray radiation 14 and a detector 16. Thesubject may be any person or object to be examined by exposure to theradiation, such as human or animal subjects in medical applications,parts and the like in industrial applications, parcels, and so forth instill further settings. The source 14 may move or may be stationary.Moreover, various X-ray sources may be employed, including conventionalX-ray tubes, as well as other types of X-ray emitters. The X-ray source14 emits an X-ray beam 18 shown as a fan beam in FIG. 1. Various beamconfigurations are known in the art and may be employed with the presentinvention.

The detector 16 is a digital detector of a type generally known andpresently in use, but with improved pixel structures as described below.The detector 16 has a surface 20 that is subdivided into an array ofdiscrete picture elements or pixels 22. As will be appreciated by thoseskilled in the art, at each pixel location 22 a pixel area includes aphotodiode and a field effect transistor (FET). These components, inaccordance with the present invention, are described in greater detailbelow. Essentially, however, X-ray radiation impacting the detectorcontacts a scintillator layer of the detector (described below) thatresults in lower energy photons impacting each photodiode. Charges atthe photodiodes are depleted by the photons, and the photodiodes may berecharged by enabling and changing the conductive state of therespective FET. In this manner, the charge depletion that eachphotodiode (pixel) location can be read, and the information used toreconstruct an image based upon the relative amount of radiationreceived at each pixel location.

The system illustrated in FIG. 1 further includes source controlcircuitry 24 which directs operation of the source 14. Data acquisitioncircuitry 26 controls enabling, readout, and other functions of thedigital detector 16. The source control circuitry 24 and dataacquisition circuitry 26 function under the overall control of systemcontrol circuitry 28. The system control circuitry typically will enablevarious imaging routines to be carried out, provide for calibration, andother system functions. Image processing circuitry 30 is provided forreconstruction of user-viewable images based upon the acquired data fromthe detector 16. Finally, viewing and interface circuitry in a station32 enables a user or operator to control the overall system functions,view images, and so forth. The particular configuration of the circuitsfor control and image reconstruction and processing may essentiallysimilar to those existing X-ray imaging systems.

FIG. 2 illustrates certain of the functional components that may belocated at the individual pixels 22 of the detector shown in FIG. 1. Inthe embodiment of FIG. 2, each pixel includes a photodiode 34 designedto receive photons resulting from impact of X-ray radiation on thedetector. The photodiode 34 is surrounded by a diode gate 36. While gate36 may partially surround the diode, in the illustrated embodiment theentire periphery of the diode is surrounded by the diode gate. Ingeneral, it is presently contemplated that the diode gate may surroundat least approximately 25% of the diode periphery, and the structurebenefits from even greater gating. A reflector/contact layer 38 isdisposed over the diode gate and around the periphery of the photodiode.As discussed in greater detail below, the reflector/contact 38 servesboth to reflect photons from around the periphery of the diode backtoward the sensing surface of the diode, and makes contact with an uppersurface of the diode to serve as a contact.

Each pixel 22 further includes a FET 40. Conductive traces are providedat the pixel location to enable the pixel to be coupled to other pixelsin the array, particularly for readout of the charge at the photodiode.In the illustrated embodiment, links 42 and 44 are provided on eitherside of the photodiode to link the photodiode to those of neighboringpixels. Similarly, conductive traces of 46 and 48 are provided to enable(change the conductive state of) the FET 40, and to readout charge fromthe pixel via the FET (e.g., by recharging the photodiode and any chargecarrying structures of the pixel). In the illustrated embodiment, trace46 is a scan line for enabling the FET, and trace 48 is a data line forreading out charge at the pixel location. As will be appreciated bythose skilled in the art, in practice, data line 48 provides forrecharging the photodiode, such that the charge added to the existingcharge at the photodiode will be proportional to the charge depletionresulting from an exposure event.

The structure illustrated in FIG. 2 is shown in partial section in FIG.3. As illustrated in FIG. 3, the pixel, and the entire array of pixels,is covered by a scintillator layer S. The scintillator serves to convertX-ray radiation received by the detector to lower energy photons thatcan be detected by the photodiode 34. The pixel structure is built upona substrate layer 50, that may be, for example, a glass. A dielectriclayer 52 is disposed on the substrate 50. Exemplary dielectrics mayinclude such materials as nitrides, glasses, and so forth. On thedielectric layer 52, a contact layer 54 is disposed. In general, contactlayer 54 will be a metal conductive layer, similar to such layers usedon conventional digital detector structures, such as moly, although manyother materials may be suitable, such as aluminum, tungsten, titanium,copper, and so forth. The diode 34, itself, is formed on the contactlayer 54, with the contact layer 54 serving as one of the contacts forcharging and recharging the diode for readout.

The diode gate 36 is a conductive layer surrounding the diode 34 andseparated from the diode by a passivation layer 56. The passivationlayer 56 may be made, for example, of silicon dioxide, silicon nitride,combinations of these, or of various other suitable materials, includingof polymers. The diode gate 36 may be made of a material similar to thatof the contact layer 54. The thickness of the passivation layer 56 iscontrolled such that the gate layer 36 essentially limits the migrationof charge (i.e., electrons) down the edge of the photodiode 34 from thereflector/contact layer 38 to the contact layer 54. Thereflector/contact layer 38 is, in the embodiment of FIG. 3, separatedfrom the diode gate layer 36 by an additional passivation layer 58,which may be essentially similar to the passivation layer 56. As will beappreciated by those skilled in the art, the passivation layers may beformed by any suitable process, such as chemical vapor deposition. Thepassivation layers essentially define insulative or dielectric layers inthe structure.

In a present embodiment, it has been found that a passivation layer 56in as range of from 500 Å to 1 μm in thickness appears to be effectiveat reducing leakage of charge from the photodiode. A thickness ofapproximately 2500 Å may be preferred. Moreover, the diode gate layer 36may, as illustrated in FIG. 3, be held at a common voltage with thereflector/contact layer 38, such as by means of a contact via 60. In apresent embodiment, for example, the reflector/contact layer 38 is heldat a voltage difference of approximately 8 volts with respect with thecontact layer 54. Voltage differences across the diode may vary,depending upon the system design, such as from approximately 4 volts toapproximately 15 volts. It should be noted, however, that the particularvoltages, thicknesses, materials, and so forth described above areprovided by way of example only. Other specifications for the componentsmay be used without departing from the scope and the intent of theinvention.

It has been found that the gate layer 36 facilitates reduction in theleakage from the photodiode, and particularly reduces the degradation ofthe photodiode performance over time. Because the gate layer is providedaround the periphery of the photodiode, leakage at this point isminimized. In accordance with other aspects of present designs, leakagecan be further minimized by reducing the surface area of the photodiode,and by modifying the configuration (e.g., shape) of the photodiodeperiphery. FIG. 4 illustrates an exemplary embodiment of a photodiodeincorporating these features.

As shown in FIG. 4, a pixel 62 having a round (circular) photodiode ofreduced surface area is illustrated. The photodiode 64 is generallysimilar in structure to that described above, but is round in periphery.A contact/reflector layer 66 is provided around the photodiode and isjoined to the photodiode for readout by contact tabs 68. It has beenfound that the use of contact tabs 68 can reduce the effective surfacearea of the photodiode covered by the contact/reflector layer 66,thereby reducing the intrusion of the contact/reflector layer into thesensitive region of the photodiode. Other aspects of the pixel structureare generally similar to that described above with reference to FIG. 2.

As shown in FIG. 5, the structure of FIG. 4 includes the substrate 50 onwhich the dielectric layer 52 and contact layer 54 are disposed. Apassivation or dielectric layer 70 is provided between thecontact/reflector 66 and the contact layer 54.

The contact/reflector layer 66 reflects photons that may impact thislayer to add to the sensitivity and data collection capabilities of theoverall structure. That is, incident radiation 18 contacting thescintillator S will include either photons that enter directly into theround sensitive region of the photodiode, as indicated by referencenumeral 72, or reflected photons 74. These reflected photons may bereflected by any portion of the contact/reflector 66, and will reboundwithin the scintillator material until they ultimately contact thesensitive area of the photodiode 64. In presently contemplatedembodiments, the reduced surface area photodiode 64 occupies from 20 to60 percent of the area of the pixel. The contact/reflector layer 66,then, increases the effective fill factor of the pixel covered by thephotodiode and contact/reflector layer 66, combined, to greater than 60%in the presently contemplated embodiments. It should also be noted thatother configurations for the photodiode may also be envisaged, such asoval shapes, and so forth. However, the circular shape illustratedminimizes the perimeter at which leakage can occur.

Moreover, in the embodiment illustrated in FIG. 5, the contact/reflectorlayer 66 serves to increase the overall capacitance of the photodiode,and more generally of the pixel location. For example, as illustrated inFIG. 6, the contact/reflector layer 66 is separated from the contactlayer 54 by a passivation layer that can be made relatively thin so asto act as a dielectric for an effective capacitor 76 defined by theselayers. It has been found that the use of such an inherent capacitor forstoring charge at the pixel location can increase the effective pixelcapacitance to in excess of 60% of the capacitance provided by a largephotodiode (i.e., a conventional structure).

The foregoing improvements to the pixel and detector design may beincorporated together in the detector. FIG. 6 illustrates such acombined and improved pixel structure. As shown in FIG. 6, thecontact/reflector 66 may, itself, serve to limit leakage from theperiphery of the photodiode. That is, when the contact/reflector 66 ispositioned in close proximity to the photodiode peripheral edge, andseparated by one or more passivation layers, the contact/reflector layer66 may itself act as a gate limiting leakage of charge around the diodeedges.

FIG. 6, then, illustrates the aforementioned improvements combined in areduced area, gated diode pixel 78 with enhanced capacitance. As notedabove FIG. 6, the reduced area photodiode 64 again has a circular shapeto reduce both its surface area and the length of its peripheral edge.The contact/reflector layer 66 overlies the region surrounding thephotodiode 64 and makes contact with the photodiode by means of contacttabs 68. Below the contact/reflector layer 66, a diode gate 36 isprovided. The diode gate 36 is electrically coupled to thecontact/reflector layer 66 by a conductive via 60 as described above.Further, below the diode gate 36, the contact layer 54 extends and isseparated from the diode gate 36 to form a capacitor to increase theeffective capacitance of the photodiode. Other structures, including theFET 40, the conductive traces 42 and 44, and the scan and data lines 46and 48 are essentially similar to those described above.

The improved pixel structure of FIG. 6, then, effectively reducesleakage and therefore degradation at the pixel location by reducing thesurface area of the photodiode, reducing the length of the peripheraledge of the photodiode, and gating the edge to reduce leakage ofelectrons along the edge, while still maintaining an elevatedsensitivity by virtue of the reflectivity of the contact/reflector layer66. The structure also increases the effective capacitance at the pixellocation by defining a capacitor between the conductive layers by meansof the intermediate passivation or dielectric layer.

It should be noted that, while the reduced surface area diode describedabove has a generally round shape, other shapes may, of course benefitfrom advantages of the designs and techniques described here. Forexample, rounded or even multi-sided (e.g., square) diodes may be formedthat still employ the gating, or reflector or capacitor conceptsdescribed above, together or in combination. Similarly, while apresently contemplated embodiment has a fill factor of approximately 60%of the surface area of the pixel, other fill factors may be used.Moreover, the reflector may occupy more surface area than the diodeitself, and the pixel still provide good sensitivity.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A photodetector formed of an array of pixels on a substrate andcovered by a scintillator, each pixel comprising: a photodiodeconfigured to receive photons and to deplete a stored charge in responseto the received photons; and a gate layer surrounding at least a portionof an edge of the photodiode, the gate layer limiting charge leakagebetween contacts of the photodiode.
 2. The photodetector of claim 1,wherein at each pixel the gate layer is electrically coupled to acontact of the photodiode and maintained at a common voltage with thecontact.
 3. The photodetector of claim 1, wherein at each pixel thediode is formed in an underlying contact layer, and the gate layer isseparated from the underlying contact layer by a passivation layer. 4.The photodetector of claim 1, wherein at each pixel the gate layer is acontact of the photodiode.
 5. The photodetector of claim 1, wherein ateach pixel the photodiode includes an upper contact layer, and the gatelayer is coupled to the upper contact layer by a via.
 6. Thephotodetector of claim 1, wherein at each pixel the gate layer ismaintained at a voltage different from a voltage level of two contactsof the photodiode.
 7. The photodetector of claim 1, wherein eachphotodiode has a substantially round periphery.
 8. The photodetector ofclaim 1, wherein each photodiode occupies a surface area of fromapproximately 20% to approximately 60% of a total surface area of therespective pixel.
 9. A photodetector formed of an array of pixels on asubstrate and covered by a scintillator, each pixel comprising: aphotodiode configured to receive photons and to deplete a stored chargein response to the received photons; a lower contact layer underlyingthe photodiode and in contact with a lower surface thereof; an uppercontact layer in contact with an upper surface of the photodiode; a gatelayer surrounding at least a portion of an edge of the photodiode, thegate layer limiting charge leakage between the upper and lower contactlayers; and a passivation layer separating the gate layer and the lowercontact layer.
 10. The photodetector of claim 9, wherein at each pixelthe gate layer is separated from the upper contact layer by anadditional passivation layer.
 11. The photodetector of claim 9, whereinat each pixel the gate layer is electrically coupled to the uppercontact layer by a via.
 12. The photodetector of claim 9, wherein ateach pixel the gate layer is maintained at a voltage different from avoltage level of the upper and lower contact layers.
 13. Thephotodetector of claim 9, wherein each photodiode has a substantiallyround periphery.
 14. The photodetector of claim 9, wherein eachphotodiode occupies a surface area of from approximately 20% toapproximately 60% of a total surface area of the respective pixel.
 15. Aphotodetector formed of an array of pixels on a substrate and covered bya scintillator, each pixel comprising: a substantially round photodiodeconfigured to receive photons and to deplete a stored charge in responseto the received photons, the photodiode occupying a surface area of fromapproximately 20% to approximately 60% of a total surface area of therespective pixel; a lower contact layer underlying the photodiode and incontact with a lower surface thereof; an upper contact layer in contactwith an upper surface of the photodiode; a gate layer surrounding atleast a portion of an edge of the photodiode, the gate layer limitingcharge leakage between the upper and lower contact layers; and apassivation layer separating the gate layer and the lower contact layer.16. The photodetector of claim 15, wherein at each pixel the gate layeris separated from the upper contact layer by an additional passivationlayer.
 17. The photodetector of claim 15, wherein at each pixel the gatelayer is electrically coupled to the upper contact layer by a via. 18.The photodetector of claim 15, wherein at each pixel the gate layer ismaintained at a voltage different from a voltage level of the upper andlower contact layers.
 19. The photodiode of claim 15, wherein the uppercontact layer and the photodiode occupy a combined surface area greaterthan approximately 60% of a total surface area of each pixel.
 20. Thephotodiode of claim 15, wherein the upper contact layer and the gatelayer form, with the lower contact layer, a capacitor for storing chargedepleted by receipt of photons by the photodiode.